According to Britannica's article on Hipparchus, " ... observed the positions of the stars and compared his results with those of Timocharis of Alexandria about 150 years earlier and with even earlier observations made in Babylonia.... He proposed precession to account for the size of the difference and he gave a value of 45 or 46 (seconds of arc) for the annual change, very close to the figure of 50.26 accepted today. ... Knowledge of precession enabled him to obtain a better value for the length of the year ... his value for the tropical year was too great by only 6 1/2 [minutes?]".
So, indeed, he had access to a long series of observations, but it doesn't answer the question of exactly how discovering precession led him to this value. Can anyone explain? To note just two of his other achievements, he produced the first star catalogue and was the first to specify position on the Earth by longitude and latitude; surely, he was a very smart fellow indeed. Andrew James
